Field of the Invention
The present invention relates to image processing for a display device.
Description of the Related Art
Image display apparatuses provided with various display devices such as a liquid crystal panel, in particular, monitors for television receivers and personal computers have been put to practical use. There exist techniques of correcting various characteristics of these display devices to display images.
For example, in a liquid crystal panel, the transmissivity or reflectance may change depending on the position on the liquid crystal panel surface, and color non-uniformity may visually be recognized accordingly. To reduce the color non-uniformity, color-non-uniformity correction processing is performed. The color-non-uniformity correction processing is processing of preparing color-non-uniformity correction values corresponding to image signal levels and positions on the liquid crystal panel surface in advance, and generating a signal (to be referred to as a “liquid crystal panel driving signal” hereinafter) used to drive the liquid crystal panel using a color-non-uniformity correction value corresponding to an input image signal.
Japanese Patent Laid-Open No. 2001-357394 discloses a technique of decreasing the data amount of color-non-uniformity correction values. That is, a reference color-non-uniformity correction value corresponding to a certain image signal level is stored. For other image signal levels, difference values from the reference color-non-uniformity correction value are stored, thereby decreasing the data amount of color-non-uniformity correction values.
After the signal level of the liquid crystal panel driving signal is changed, a certain time is needed until the signal level of the liquid crystal panel driving signal actually reaches a desired signal level. This time is called a “response speed.” There exist liquid crystal panels whose response speeds are not sufficiently high. In addition, since the response speed changes in accordance with the transition of the signal level between frames, problems such as afterimage generation and color change in moving image display remain.
To improve the response speed of a liquid crystal panel, there has been proposed so-called overdrive correction processing of comparing an image signal displayed in the preceding frame with an image signal to be displayed in the next frame and correcting the liquid crystal panel driving signal in accordance with the comparison result (Japanese Patent Laid-Open No. 04-302289).
When tracking a moving object displayed on a hold-type liquid crystal panel (to be referred to as “tracking sight” hereinafter), a motion blur according to the light output period is observed. There has been proposed a technique of doubling the frame rate of an input image signal (for example, from 60 Hz to 120 Hz) to generate subframes (frame rate doubling) and outputting one subframe image as a black image or dark image. According to this technique, the light output period of the hold-type liquid crystal panel is shortened, and the motion blur can be reduced. In, for example, Japanese Patent Laid-Open No. 11-126050 (literature 3), the continuous light-emitting period or effective light-emitting period is limited from at least 30% to not more than 70% between subframes, thereby reducing the motion blur or afterimage that makes the motion of an object unnatural.
According to the technique disclosed in literature 3, display with a light/dark difference being given between subframes is performed (to be referred to as “light and dark display” hereinafter), thereby reducing unnaturalness of a motion of an object. On the other hand, since the signal level changes between the subframes of a single frame, the influence of the response speed of the liquid crystal panel readily becomes large as compared to a case in which only frame rate doubling is performed.
According to color-non-uniformity correction processing, even if image signals have the same signal level, liquid crystal panel driving signals of different signal levels are generated in accordance with positions on the liquid crystal panel surface. For this reason, when light and dark display and color-non-uniformity correction processing are combined, the influence of the response speed changes depending on the position on the liquid crystal panel surface. Hence, appropriate color-non-uniformity correction processing may be impeded by the influence of the response speed, and color non-uniformity may be observed. Although the color non-uniformity can be reduced by reducing the influence of the response speed by overdrive correction processing, as a matter of course, a frame memory for storing the image signal of the preceding frame is necessary, and the memory amount increases.